Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School 2013 Design and Modeling of Very High- Efficiency Multijunction Solar Cells Indranil Bhattacharya Follow this and additional works at the FSU Digital Library. For more information, please contact [email protected] THE FLORIDA STATE UNIVERSITY FAMU-FSU COLLEGE OF ENGINEERING DESIGN AND MODELING OF VERY HIGH-EFFICIENCY MULTIJUNCTION SOLAR CELLS By INDRANIL BHATTACHARYA A Dissertation submitted to the Department of Electrical and Computer Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy Degree Awarded: Summer Semester, 2013 Indranil Bhattacharya defended this dissertation on June 20, 2013. The members of the supervisory committee were: Simon Y. Foo Professor Directing Dissertation Anke Meyer-Baese University Representative Jim P. Zheng Committee Member Petru Andrei Committee Member Hui Li Committee Member The Graduate School has verified and approved the above-named committee members, and certifies that the dissertation has been approved in accordance with university requirements. ii Dedicated to my loving parents (Mrs. Sipra Bhattacharya and Mr. Benoy Bhattacharya), brother (Mr. Rudranil Bhattacharya), professors & teachers, friends & well wishers and all who have inspired me to get to where I am today. Thank you all. iii ACKNOWLEDGEMENTS I would like to thank my wonderful parents and younger brother for their love and continued support. Nothing would have been possible without my mother’s prayers and father’s inspiration. My mother has stood by me through all odds in life. My father has made me ambitious and my younger brother has been my biggest supporter. I would like to convey my sincere gratitude to my major professor Dr. Simon Y. Foo for his encouragement, invaluable advice and support throughout this process. His invaluable advice, persistent support and patience have helped me achieve the research objectives. I thank him from the core of my heart for standing by me through all highs and lows of the process and guiding me towards the right direction. I am grateful to the other members of my committee, Dr. Jim P. Zheng, Dr. Petru Andrei, Dr. Hui Li and Dr. Anke Meyer-Baese for their invaluable advice, time and support. Thank you all. I would like to thank my friends and well wishers for their encouragement, support and extending their helping hands whenever needed. iv TABLE OF CONTENTS List of Tables ............................................................................................................................... viii List of Figures ..................................................................................................................................x Abstract ........................................................................................................................................ xiii 1. PHOTOVOLTAICS: AN INTRODUCTION .........................................................................1 1.1 Photovoltaics: Mechanism, Advantages, Challenges and Different Generations ..........1 1.1.1 Mechanism .........................................................................................................1 1.1.2 Advantages of Photovoltaic Technology ...........................................................3 1.1.3 Challenges of Photovoltaic Technology ............................................................3 1.1.2 Different Generations of Solar Cell Technology ...............................................4 1.2 Motivation of High-Efficiency Multijunction Research ................................................5 1.3 Organization of Work ...................................................................................................7 2. DEVICE PHYSICS OF SOLAR CELLS ..............................................................................10 2.1 Introduction ..................................................................................................................10 2.2 Fundamental Properties and Physics of Semiconductors .............................................11 2.2.1 Semiconductor Crystal and Energy Band Structure ........................................11 2.2.2 Valence-Band and Conduction-Band Densities...............................................13 2.2.3 Equilibrium Carrier Concentrations .................................................................13 2.2.4 Light Absorption ..............................................................................................16 2.2.4.1 Direct and indirect bandgap semiconductors .....................................16 2.2.5 Recombination .................................................................................................20 2.2.6 Carrier Transport ..............................................................................................23 2.2.7 Semiconductor Equations ................................................................................26 2.2.8 Minority Carrier Diffusion Equation ...............................................................26 2.2.9 Diode Electrostatics p-n junction .....................................................................27 2.3 Solar Cell Fundamentals .............................................................................................30 2.3.1 Solar Cell Boundary Conditions ......................................................................30 2.3.2 Generation Rate ...............................................................................................31 2.3.3 Interpreting Solar Cell Characteristic ..............................................................31 2.3.4 Properties of Efficient Solar Cells ...................................................................33 3. LOSS REDUCTION MECHANISMS ..................................................................................35 3.1 Introduction ..................................................................................................................35 3.1 Optical Design of Solar Cells ......................................................................................35 3.2.1 Light Trapping .................................................................................................35 3.2.2 Reduction of First Reflection: Anti Reflective Coating ..................................37 3.3 Surface Recombination Loss and Reduction ...............................................................38 3.3.1 Surface Recombination ....................................................................................38 3.3.2 Surface Passivation of Non-Contacted Areas .................................................40 3.3.3 Surface Passivation of Metal-Contacted Areas ................................................41 3.3.4 Front Surface Passivation: Homojunction vs. Heterojunction Design ............42 3.3.5 Homojunction Emitter Design and its Relation to Surface Passivation ..........43 3.4 Bulk Recombination Losses and Reduction ................................................................44 v 3.4.1 Electronic Description of Grain Boundaries ....................................................45 3.4.2 Effect of Grain Boundaries on Majority Carrier Transport .............................46 3.4.3 Effect of Grain Boundaries on Carrier Recombination ...................................47 3.4.4 Designs to Overcome Low Diffusion Lengths ................................................48 3.4.4.1 Graded base design ...........................................................................48 3.4.4.2 p-i-n cell design.................................................................................49 3.4.4.3 Parallel multijunctions ......................................................................50 4. HIGH-EFFICIENCY III-V COMPOUND SEMICONDUCTOR MULTIJUNCTION SOLAR CELLS ....................................................................................................................51 4.1 Introduction ..................................................................................................................51 4.2 Design Physics of III-V Multijunction Solar Cells ......................................................52 4.2.1 Wavelength Dependence of Photon Conversion Efficiency ............................52 4.2.2 Effective Spectrum Splitting ............................................................................52 4.2.3 Cell Quantum Efficiency (QE) and Current Density (Jsc) ...............................53 4.2.4 Current Matching and Top Subcell Thinning ..................................................54 4.2.5 Current Matching Effect on Fill factor and Open-Circuit Voltage ..................55 4.2.6 Spectral Effects ................................................................................................55 4.2.7 Anti reflective Coating (ARC) Effects ............................................................56 4.2.8 Concentrator Cells ...........................................................................................56 4.3 III-V Multijunction Solar Cells: Lattice-Matched and Lattice-Mismatched ...............57 4.3.1 Lattice-Matched (LM) Systems .......................................................................57 4.3.2 Lattice-Mismatched (LMM) Systems ..............................................................59 4.3.2.1 Inverted metamorphic multijunction solar cells ...............................60 4.4 Cell Interconnects, Window and Back Surface Field layers........................................61